An equilibrium mixture of xylenes contains about 24% para-xylene (PX), 56% meta-xylene (MX), and 20% ortho-xylene (OX). PX is relatively high value as compared with MX and OX, and it is desirable to isomerize OX and/or MX to PX, such as isomerizing a PX-lean stream to equilibrium for PX recovery. It is an active area of research.
Typically xylene streams found in chemical or petrochemical plants also contain ethylbenzene (EB). Conventional isomerization technologies operating at high temperatures (e.g.: 400° C.) in vapor phase isomerize the xylenes and dealkylate EB to benzene. Other vapor-phase isomerization technologies convert EB to xylenes in addition to xylenes isomerization. There are also liquid-phase isomerization technologies. Conventional isomerization technologies typically produce significant amounts (>0.5 mol %) of byproducts such as benzene and A9+ (aromatic hydrocarbons having 9 or more carbon atoms). Thus, it is necessary to install a topping and/or a tailing distillation column to reduce the byproducts concentrations. In many situations, installing new distillation columns would not be feasible due to economic and/or physical constraints. Most isomerization technologies also require high hydrogen partial pressure to maintain the catalyst activity, which makes the process arrangement complex and expensive.
U.S. Pat. No. 6,180,550 teaches ZSM-5 useful in the liquid phase isomerization of xylene. The zeolite used has a SiO2/Al2O3 ratio of less than 20.
U.S. Pat. No. 6,448,459 teaches isomerization without hydrogen in a liquid phase diluted with toluene used as desorbent in a simulated moving bed adsorptive separation unit. The catalyst used in the liquid phase isomerization is said to be zeolitic, for example ZSM-5, and in the example it is specified that there is no hydrogen.
U.S. Pat. No. 6,872,866 teaches a two stage, liquid or partially liquid phase isomerization process using a zeolitic-based catalyst system preferably based on zeolite beta and on pentasil-type zeolite. This patent also sets forth numerous examples of prior art catalyst systems, including ZSM-5.
U.S. Pat. No. 7,244,409 teaches small crystallite ZSM-5 which may be used for isomerization reactions.
U.S. Pat. No. 7,371,913 teaches a ZSM-5 mole sieve further comprising Ga is used as an isomerization catalyst to provide an increased amount of PX in the liquid phase in the substantial absence of H2. The amount of H2 present is stated to be less than 0.05, preferably less than 0.01, mole H2/mole feed.
U.S. Pat. No. 7,495,137 teaches a two-stage isomerization system, the first zone operating in the absence of hydrogen (as in the above patent) using a platinum-free catalyst and the second zone using a catalyst comprising a molecular sieve and a platinum-group metal component. The catalyst in the first zone is preferably a Ga-MFI-type zeolite and it is preferred that the catalyst for the first zone has a Si:Al ratio greater than about 10.
U.S. Pat. No. 7,592,499 teaches a multi-stage process for co-producing PX and styrene from a feed of hydrocarbons comprising xylenes and EB. In the first stage, PX is separated from the feed by means of a simulated moving bed adsorptive separation column to produce a raffinate comprising EB, OX, and MX. Next, EB in the raffinate is dehydrogenated to styrene. Eventually a stream containing unconverted EB, MX, and OX is obtained and contacted with an isomerization catalyst to preferably in the liquid phase. The catalyst is zeolitic, such as ZSM-5.
U.S. 2009-0182182 teaches a two-stage isomerization process, the first stage in the liquid phase in the substantial absence of H2 to obtain an intermediate stream. In the second stage, the intermediate stream is mixed with a stream rich in naphthene, and contacted with an isomerization catalyst. By “substantial absence of H2” is meant no free hydrogen is added to a feed mixture and any dissolved hydrogen from prior processing is substantially less than about 0.05 moles/mole of feed. The first isomerization catalyst includes a molecular sieve, typically an aluminosilicate having a Si:Al2 ratio greater than about 10. In the example given, a Ga source is used to make the catalysts for both the first and second isomerization steps.
U.S. Publication No. 2010-0152508 teaches a process for isomerization that is at least partially in the liquid phase and includes a step of removal of C9 aromatic hydrocarbons from a feedstream including C8 and C9 aromatic hydrocarbons.
The present inventors have discovered a xylenes isomerization technology to provide a product enriched in PX when compared with the feedstream to the process. In embodiments the process takes a PX-lean feedstream to produce a product having equilibrium or near equilibrium xylenes. In embodiments the process produces very low level of by-products (such as <0.3 wt. %). Thus there is no need for additional distillation columns. Furthermore, the technology can operate without the presence of any hydrogen or with only low ppm levels of dissolved hydrogen, making it a simple and cost-effective process.